Fe3O4@SiO2@CSH+VO3− as a novel recyclable heterogeneous catalyst with core–shell structure for oxidation of sulfides

Iron nanoparticles, with low toxicity and many active sites, are among the materials that not only reduce waste along with green chemistry but also increase the separation power and recover the catalyst from the reaction environment. In this study, first, the surface of iron nanoparticles was silanized, and in the next step, the complex of chitosan HCl.VO3 was placed on the surface of Fe3O4 (Fe3O4@SiO2@CSH+VO3−). This nanocatalyst is a novel, recoverable, and potent nanocatalyst with high selectivity for the oxidation of sulfides to sulfoxides. Various physicochemical techniques such as IR, XRD, TGA, SEM, EDX, mapping, TEM, and VSM were used to affirm the well synthesis of the catalyst. Oxidation of sulfides in the presence of hydrogen peroxide as a green oxidant and in ethanol was catalyzed by the Fe3O4@SiO2@CSH+VO3−. All sulfoxides were achieved with high efficiency and in a short time. The notable privileges of this method include facile and economic catalyst synthesis, proper catalyst durability, great performance, simple catalyst isolation, good recovery capability, at least up to 5 times without an index drop in catalytic power.


Material and methods
The materials used in this study include FeCl 3 , NaOH, FeCl 2 , EtOH, CH 2 Cl 2 , tetraethyl orthosilicate (TEOS), chitosan (CS), HCl and NH 4 VO 3 , all of which were purchased from Merck without purification.The FT-IR spectra were recorded using a Shimadzu IR-470 spectrophotometer.TGA spectra were obtained using the STA504 device in the temperature range of 25-1000 °C, with the temperature increasing by 10 °C every minute during the analysis.Results from EDX-mapping analyzes were recorded with a Brucker TESCAN equipped with a SAMX Detector.FESEM images were acquired using a TESCAN MIRA3 at various magnifications.The magnetic strength of the catalyst was determined using a VSM apparatus from Magnatis Kavir Kashan Company.TEM images were captured using a CM 120 instrument from the Netherlands with a maximum voltage of 100 KV.X-ray diffraction patterns were prepared using a JEOL-JDX-8030 instrument (30 KV, 20 mA).

Synthesis of Fe 3 O 4 NPs
Iron nanoparticles were synthesized based on the method mentioned in scientific reports 15,19 .

Silanization of the surface of Fe 3 O 4 NPs
To silanize the surface of iron nanoparticles, 0.5 g of iron nanoparticles were dispersed in 25 mL of a mixture containing water and ethanol in a volume ratio of 1:8.Then, 1 mL of ammonia solution was added to the dispersion.After a few minutes, 1 mL of tetraethyl orthosilicate (TEOS) was added to the mixture, which was then stirred for 12 h at room temperature.Upon completion of the reaction time, the nanoparticles were collected using a strong magnet, washed with water and ethanol, and finally dried in an oven at 50 °C.

Immobilization of chitosan hydrochloride on the surface of Fe 3 O 4 @SiO 2
Chitosan hydrochloride was obtained by dissolving chitosan in 20 cc of a 1% hydrochloric acid solution under stirring at 1000 rpm.Subsequently, 0.5 g of silanized iron nanoparticles was added to this solution, and the mixture was refluxed for 24 h.The synthesized Fe 3 O 4 @SiO 2 @CS.HCl was easily collected using a strong magnet and rinsed at 60 °C.

Formation of Fe
To synthesize the final catalyst, 0.1 g of ammonium metavanadate (NH 4 VO 3 ) was added to 0.5 g of Fe 3 O 4 @SiO 2 @ CS.HCl nanoparticles in ethanol.The mixture was refluxed at room temperature for 24 h.After the desired time, the resulting nanoparticles were easily collected using a strong magnet and washed several times with ethanol and water.Finally, they were dried at 50 °C to produce Fe 3 O 4 @SiO 2 @CSH + VO 3 − (Fig. 1).

A general procedure for the synthesis of sulfoxides
A mixture of 30% hydrogen peroxide (0.4 mL) and sulfide (1 mmol) was added to a round-bottom flask containing Fe 3 O 4 @SiO 2 @CSH + VO 3 − (0.05 g).The resulting mixture was vigorously stirred in ethanol at ambient temperature.The progress of the reaction was monitored by TLC.Upon completion of the reaction, the nanocatalyst was easily removed using a powerful external magnet.The products were then extracted by adding water and ethyl acetate.After evaporation of the organic solvent, the desired sulfoxides were obtained with high purity (Fig. 2).

Catalyst characterization
Figure 3 shows the FT-IR curves of Fe 3 O 4 NPs (blue curve), Fe 3 O 4 @SiO 2 (red curve), Fe 3 O 4 @SiO 2 @ CSH + VO 3 − (green curve).In blue curve, peaks appearing at 440.1 and 621.5 cm −1 can be attributed to Fe-O in cubic structure of Fe 3 O 4 NPs.The stretching vibration of OH groups in iron nanoparticles appeared at 3415.3 cm −1 .In all the curves, the bending vibration of OH (water) has appeared in the region of 1640-1645 cm −1 .In the FT-IR of Fe 3 O 4 @SiO 2 NPs (red curve), in addition to the peaks related to Fe-O in the range of 446.5-616.3cm −1 , the stretching vibration of Si-O has appeared at 1054.4 cm −1 .Also, OH stretching vibrations related to TEOS (Tetraethyl orthosilicate) and Fe 3 O 4 NPs are observed in 3413.3 and 3462.5 cm −1 , respectively.In FT-IR of Fe 3 O 4 @SiO 2 @CSH + VO 3 − (green curve), peaks appearing at 464.4 and 631.9 cm −1 indicate the presence of Fe-O in the nanocatalyst structure and the successful synthesis of iron nanoparticles 15,19 .The presence of V-O can be confirmed by the vibrational frequency at 521.5 cm −1 , also three frequencies in the regions of 794.2, 851.4 and 935 are assigned to polymeric vanadate groups 53 .The two peaks appearing at 1091 to 1220 cm −1 are related to the Si-O bond, which indicates the synthesis of the core-shell structure of silanized Fe 3 O 4 @SiO 2 NPs 54 .The presence of chitosan in the nanocatalyst structure is proved by the strong symmetric stretching frequency of the N-H group at 1409.5 cm −114,55 .Based on the Fig. 3 stretching absorptions of methylene and methyl groups appeared in the region of 2850 and 2940 cm −1 .The vibration observed at 1629.6 cm −1 can be related to the bending frequency of the hydroxyl group in the structure of chitosan, water and Fe 3 O 4 NPs 15,19,35 .The stretching vibrations related to the OH groups in iron nanoparticles appeared at 3419.2 cm −1 and the OH group of water molecule in the structure of nanocatalyst appeared at 3425.5 cm −1 .
X-ray diffraction technique was used to determine the crystalline structure of the synthesized nanocatalyst (Fig. 4).The peaks appearing at 2theta 15.21, 18.41, 23.86, 28.51, 33.46, 34.41, 49.86, 51.51, 60.51, 65.81, and  68.16 indicate the excellent binding of ammonium vanadate on the surface of silanized magnetic nanoparticles 56 .Diffraction peaks appearing at 30.41 (220), 35.51 (311), 43.91 (400), 53.91 (422), 57.71 (511) and 63.36 (440) confirms the cubic structure for iron nanoparticles (JCDPS card no, 19-0629) 57 .In order to obtain the size of the particles, the Debye Scherrer Eq. ( 1) was used.After calculations, the particle size was obtained in the range of 15 to 72 nm.here, D is the crystallite size, K is the shape factor, calculated for spherical particles is 0.98, K = 1.54 A• for Cu and β is full width at half maxima of the highest peak in radian.
(1) The morphology and shape of synthesized nanoparticles were scrutinized by FESEM analysis.FESEM illustrations of Fe 3 O 4 NPs, silanized iron nanoparticles and heterogeneous nanocatalyst are shown in Fig. 5a-c.Nanoparticles in all photos are almost uniformly distributed and have a relatively spherical structure with a size of 40-80 nm.After binding the chitosan-HCl.VO 3 complex on the surface of Fe 3 O 4 @SiO 2 , there was no change in the morphology of the nanoparticles.TEM analysis was used to acknowledge the core-shell shape of magnetic nanocatalyst and estimate the exact size of the particles.Figure 5d reveals the core-shell structure of nanoparticles and the average particle size is betwixt 30-40 nm. Figure 5e displays the TEM image after 5 th use of the catalyst.Based on this picture the structure of the catalyst was maintained after several runs.Also, the catalyst was analyzed by FESEM after 5 consecutive usages, and as can be seen, the morphology of the nanocatalyst has been completely preserved (Fig. 5f).
To confirm the successful synthesis of the nanocatalyst and to verify the presence of all the essential elements in its structure, the Energy Dispersive X-ray (EDX) technique was employed.(Fig. 5g).The EDX image illustrates the successful synthesis of nanoparticles and the excellent dispersion of all key elements such as Fe, Si, O, N, C, V, and Cl within the structure of modified iron nanoparticles with CS.VO 3 .HCl.Additionally, mapping analysis confirmed the proportional presence of Fe, N, O, C, V, Cl, and Si in the structure of the nanocatalyst (Fig. 5h).Furthermore, vanadium is effectively positioned on the surface of nanoparticles modified with chitosan.
The VSM pattern of Fe 3 O 4 @SiO 2 @CSH + VO 3 − was displayed in Fig. 6.The obtained magnetic strength is 20 emu/g, which due to covering the surface with TEOS and chitosan, the obtained magnetic strength indicates the easy separation of the nanocatalyst from the reaction mixture.
The thermal stability of the synthesized nanocatalyst was assessed using the TGA technique over a temperature range of 30-1000 °C.(Fig. 7).The TGA curve reveals several stages of weight reduction.Approximately 2%  www.nature.com/scientificreports/ of weight loss is observed in the region of 30-200 °C, which can be attributed to the removal of organic solvents and moisture absorbed in the nanocatalyst structure 20,58 .A 10% weight loss, attributed to organic groups such as chitosan and inorganic groups such as vanadium attached to the surface of iron nanoparticles, occurs in the temperature range of 200-400 °C59 .Moreover, within the temperature range of 400-1000 °C, a weight loss of 4% may indicate the decomposition of the silanized nanoparticles' structure.Specific surface area (18.6324 ± 0.2385 m 2 /g), pore volume (0.060247 cm 3 /g) and pore size (129.3378Å) were calculated by Brunauer-Emmett-Teller (BET) technique.According to the adsorption and desorption diagram, the synthesized nanocatalyst exhibits a type IV isotherm, indicative of the mesoporous structure of the nanoparticles (Fig. 8).

Catalytic evaluation
After identifying and confirming the structure of the nanocatalyst introduced in this study, the catalytic efficiency of Fe 3 O 4 @SiO 2 @CSH + VO 3 − in the preparation of sulfoxides was evaluated (Table 1).The reaction of benzyl phenyl sulfide with the oxidant (H 2 O 2 ) was chosen as the selected reaction to optimize the reaction conditions.The impact of key variables such as the amount of catalyst, type of solvent, and amount of oxidant on the reaction www.nature.com/scientificreports/process was thoroughly investigated.Initially, sulfide oxidation was examined in the absence of catalyst and oxidant, resulting in no sulfoxide formation.Furthermore, the reaction was conducted with the model in the presence of catalyst defects and with the oxidant in ethanol solvent, resulting in approximately 20% product formation.The effects of varying the amount of catalyst and the amount of hydrogen peroxide on the product percentage in ethanol solvent were then studied.First, the amount of oxidant was optimized.Amounts of 2-4 mmol of H 2 O 2 were examined, and the best results were observed with 4 mmol of H 2 O 2 (Table 1, entry 13).To www.nature.com/scientificreports/optimize the amount of catalyst, 0.025 to 0.07 g of magnetic nanocatalyst were checked (Table 1, entries 10-14).
Based on the results, the best efficiency (98%) was obtained in the presence of 0.4 mL of hydrogen peroxide and 0.05 g of catalyst in ethanol solvent and in 1 h (Table 1, entry 13).Next, the synergistic effect of different parts of the catalyst and the effect of the structure on the oxidation of sulfide were investigated.As can be deduced from the Table 1, entries 21-23, synergistic effects and morphology are not effective on the sulfide oxidation.The morphology of different parts is almost spherical (according to FESEM results).Sulfide was oxidized (98%) only in the presence of the final catalyst Fe 3 O 4 @SiO 2 @CSH + VO 3 − .After determining the optimal amount of oxidant and catalyst, the influence of solvent polarity on the extent of sulfide oxidation was examined.Solvents with varying polarity, including acetonitrile, water, DMF, toluene, chloroform, and solvent-less conditions, were evaluated.Interestingly, in all cases, the desired sulfide was oxidized with lower yields compared to ethanol solvent.Additionally, increasing the amount of H 2 O 2 led to the exclusive formation of sulfoxide without the formation of sulfone product.
After obtaining the optimized conditions, various aromatic and aliphatic sulfides were oxidized in the presence of hydrogen peroxide and Fe 3 O 4 @SiO 2 @CSH + VO 3 − in ethanol (Table 2).The presented catalyst exhibited remarkable performance for the oxidation of sulfides, with the desirable sulfoxide prepared with high efficiency and in a relatively short time in all cases.It is noteworthy that aromatic sulfides containing electron-withdrawing groups yielded products in longer reaction times and with lower efficiency compared to aromatic sulfides with electron-donating groups.Additionally, the method demonstrated exceptional chemoselectivity in the oxidation of sulfides, where even in the presence of sensitive alcohol groups, only the sulfide was oxidized while the alcohol group remained intact.Furthermore, 2-(benzylthio)-1H-benzimidazole, as a heterocyclic sulfide, produced the desired sulfoxide with excellent yield.
The details of sulfide oxidation in the presence of Fe 3 O 4 @SiO 2 @CSH + VO 3 − are as follows (Fig. 9).In the initial step, H 2 O 2 can bind to vanadium on the catalyst surface, resulting in the removal of one molecule of water.Subsequently, the sulfide, acting as a nucleophile, attaches to the oxygen atom bound to the vanadium, ultimately leading to the formation of the sulfoxide 62 .
Reusability of the Fe 3 O 4 @SiO 2 @CSH + VO 3 − One of the prominent objectives in green chemistry is the utilization of nanocatalysts that can be easily recovered from the reaction medium.Therefore, the recovery capability of the nanocatalyst in the sulfide oxidation reaction under optimized conditions was examined using a model reaction.After the formation of the sulfoxide, the catalyst was removed from the environment using a magnet, washed with water and ethanol, and then dried for subsequent reactions.As shown in Fig. 10, after 5 consecutive uses, a slight decrease in catalytic activity was observed, which could be attributed to contamination of the nanocatalyst surface.EDX and XRD analyses were performed on the nanocatalyst after 5 consecutive uses.It is noteworthy that all the main elements can be observed in the EDX Catalyst image after the fifth use (Fig. 11a).As shown in Fig. 11b, the structure of the nanocatalyst remains intact with no significant changes observed.In the XRD spectrum, all the elements are present, with only variations in the intensity of the peaks, either decreasing or increasing.

Leaching test
Furthermore, a leaching test was conducted to confirm the heterogeneous nature of the prepared Fe 3 O 4 @SiO 2 @ CSH + VO 3 − catalyst for the oxidation of sulfides.In this test, benzyl phenyl sulfide was chosen as the selected reaction under optimal conditions.Halfway through the reaction time (30 min), the reaction was halted (yielding 51%), and the catalyst was separated from the reaction mixture using a magnet.Subsequently, the mixture was allowed to continue in the absence of the catalyst under stirring, and after a certain period, only a negligible increase of about 2% in the yield of sulfoxide was observed.This minimal change in the product quantity confirms the heterogeneous nature of the catalyst and indicates the absence of vanadium leaching into the reaction medium.

Comparison of catalyst efficiency
To compare the efficiency of the introduced nanocatalyst, several scientific reports were reviewed, all of which involve iron nanoparticles modified with different metals or linkers.The results are summarized in Table 3.As shown in Table 3, the synthesized magnetic nanocatalyst in this study exhibits superiority over other catalytic systems in terms of reaction time and efficiency.Additionally, it is comparable in terms of the amount of oxidant and solvent used.Specifically, the oxidation of benzyl phenyl sulfide was compared.

Conclusion
In summary, this research presents the synthesis of a novel heterogeneous magnetic nanocatalyst containing a vanadium-chitosan complex through a simple and cost-effective method.The nanocatalyst exhibited selective conversion of sulfides into sulfoxides under mild conditions with high yield.Notably, minimal leaching of vanadium from the catalyst surface was observed, which is environmentally beneficial.Comprehensive analyses including IR, XRD, TEM, FESEM, EDX, mapping, TGA, and VSM confirmed the successful synthesis of the nanocatalyst.This procedure offers several advantages such as easy catalyst preparation, no requirement for

Table 3 .
Comparison of the performance of the synthesized magnetic nanocatalyst with several other catalytic systems.